Accurate heart rate measurements may improve the utility of wrist-worn devices that measure the movements and activities of a user and provide activity points in response. For some work outs, the activity may involve substantially static movements that work mainly against gravity but are nonetheless strenuous. Examples include yoga, weight lifting, and other isometric exercises. Utilizing an accelerometer as the sole sensing element for these types of activities may result in an underestimating of the activity if the recorded movement is minimal compared to other activities that involve larger movements such as running or dancing in which the heart rate is strongly associated with the intensity of the movement. The addition of heart rate to the activity estimation algorithms greatly improves the range of activities in which an accurate estimate of activity occurs.
When measuring heart rate for sports-related activities and other types of activities, it is desirable that the heart rate sensor is portable and non-invasive. Existing technologies that may be used to measure heart rate, such as electrocardiography (ECG), may not be suited for some types of activities. Although some existing technologies to measure heart rate have been incorporated into devices that may be worn by a user, these existing technologies may require a user to actively select a button at the device to initiate the heart rate measurement. Moreover, existing device that may be worn by a user may require the device to be in intimate contact with the skin of the user in order to perform an accurate heart rate measurement. Such requirements may not be feasible in order to measure the heart rate of a user contemporaneous with performance of an activity by the user. In addition, existing devices that may be worn by users, such as a chest strap may be uncomfortable.
A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.